1. Field of the Invention
This invention relates to a load moment indicator system for a material handling device attached to a boom, and more particularly to a load moment indicator which warns the operator when the maximum load lifting capacity is being or has been reached, so that toppling or structural failure is avoided. It will be understood that depending on design, certain equipment will structurally fail before toppling, and vice versa.
Although the following description is with reference to a truck mounted crane where the load is transferred to the boom via a load line, the invention has application in other material handling apparatus having load lifting means where maximum load lifting capacity is a concern. The invention concepts can be used, for example, in fork lift trucks, personnel lift or work platforms, grapples, augers, clamshells or buckets, electromagnet attachments fixed to the load, etc. Similarly, the invention applies to self-propelled and non-self-propelled machines with or without outriggers, machines having fixed or telescoping booms, and machines having more than one lift cylinder, for example, articulating booms or booms having dual lift cylinders. Sensors in accordance with the invention can thus be installed in any cylinder supporting the structure and thus the load. For example, a linear actuator or a cylinder which is pneumatically actuated could also utilize the strain sensor of the present invention.
2. Description of the Related Art
In a crane installed on a base, as for example a truck, there is always a concern that if too great a load is lifted, the crane will topple over due to the large moment created around the axis of rotation of the crane boom, or the crane will structurally fail. The moment created is a function of the boom length, the boom angle, and the load being lifted. As might be expected, where a telescopic boom is used, the moment created during the lifting of a particular load can change quite rapidly as the boom is telescoped inwardly or outwardly, while simultaneously being rotated about its axis and thereby changing the boom angle.
Accordingly, it is very important that the crane operator be aware of these parameters in order to ensure that the crane does not exceed maximum lifting capacity. In order to assist the crane operator in performing this function, a number of indicator systems have been created to help identify when a critical crane configuration is reached. These indicator systems are commonly referred to as Load Indication Systems, Overload Protection Devices, Safe Load Indicators, Rated Capacity Indicators, and Load Moment Indicator (LMI) Systems.
The above systems generally consist of some but not always all of the following: means for detecting the weight of the load being lifted, means for determining the boom length and angle, and means providing rotational information. All of these factors should take into account all permissible loads on the system but, as above noted, the computation of maximum loads on a continuous basis is difficult to accomplish even with sophisticated programming.
In theory, based on the boom length and angle information, a load radius from the center line of the rotation of the boom to the hook block can be calculated. A load chart is then created which shows a maximum lifting capacity for each configuration of a particular load radius and boom length. Therefore, by comparing the weight of the actual load being lifted with the maximum lifting capacity for the appropriate crane, a crane operator can determine if that capacity is being reached or exceeded and take corrective action to preclude the toppling over or structural failure of the crane.
In Load Indication Systems, the crane operator must determine the crane configuration and then go to the load chart to determine the maximum lifting capacity. This manual process takes a great deal of time, relies heavily on the operator, and is not very useful in situations where the crane configuration is rapidly changing. In LMI systems, on the other hand, the crane configuration is automatically determined, and the maximum lifting capacity based on that configuration calculated on a continuing basis. However, calculating the load on a continuous basis for every point in space in terms of maximum lifting capacity creates a computational load which is difficult to manage.
Currently, a majority of the LMI systems commercially available use either pressure sensors in the lift cylinder, a tensiometer in the load line, a chain link style load cell at the dead end of the load line, or a boom lifting cable. Other LMI systems utilize either a sheave pin style load cell, or a shackle style load cell to measure the load. The last two load measuring techniques are most prevalent in systems that provide a read out of the weight of the load. Each of the above-mentioned techniques for measuring load has a number of disadvantages which will be described hereinbelow.
In most telescoping booms, at least one cylinder is used to raise and lower the boom. Thus, measuring the load as it is transferred down through the cylinder is a commonly known technique. In this system, a pressure transducer or transducers are attached to the cylinder to measure the pressure within the cylinder. At first, these systems only measured the pressure on the piston side of the cylinder. This proved unacceptable for two reasons. First, every maximum lifting capacity as determined by the load chart does not cause the same pressure to be generated in the lift cylinder. Secondly, moving the boom with a load suspended in the air generates significantly different pressures then when the boom is held stationary and the load is lifted with the winch.
The first problem can be resolved by adding length and angle sensors to the crane, and using the inputs from those sensors to determine a maximum lifting capacity for a particular machine configuration. However, the solution to the second problem has been more elusive. In many of the pressure sensing systems, a second pressure sensor on the rod side of the hydraulic cylinder is employed. Subtracting the rod side signal from the piston side signal would, in theory, eliminate the error. However, this solution cannot correct the non-linearities created by the movement of the piston head in the cylinder. Friction, unequal volumes of oil, and oil viscosity changes all contribute to the non-linearities. In addition, having two sensors doubles the possibility of sensor error and increases the number of system components that can fail. Finally, another drawback of the above load sensing system is that the pressure must be sensed on the cylinder side of the safety holding valves. This creates the possibility that an uncontrolled descent of the boom may occur if the hydraulic line or sensor is damaged.
A tensiometer operates by passing the load bearing cable through a series of sheaves which are designed to measure the force applied to the middle sheave. Based on this information, the weight of the load being lifted can be determined. Tensiometers have three major shortfalls. First, the load bearing cable reacts to the load being applied just like a spring would. Thus, a lag time associated with calculating the load is increased every time the load line passes over a cable sheave. As the number of sheaves is increased, the lag time in determining the load is also increased. Secondly, the tension in the section of the cable which is being measured by the tensiometer is dependent on the number of lines which are reeved around the hook block and the sheaves. The system is thus dependent on the operator to input the correct configuration of the hook block and sheaves. If the operator makes a mistake, he will get erroneous data. Thus, the potential for exceeding the maximum lifting capacity of the crane, without receiving a warning, exists. Finally, when using a tensiometer, every time a cable passes around a sheave it causes wear and tear on the cable and therefore reduces the expected life of the cable.
Load shackles are heretofore probably the most accurate method for determining the load being lifted by a crane. However, since the load shackle is connected to the hook of the crane, it is extremely difficult to get the signal generated by the load shackle back to the operator. Radio transmission is the only practical solution to this problem, but this is a prohibitively expensive design option for an LMI system. In addition, the shackle also increases the overall length of the hook block assembly.
Chain link style load cells are often the method of choice for lattice boom cranes. However, most other crane styles do not have a place in the structure where a load bearing cable is terminated unless an even number of lines are used with the hook block. For telescopic cranes, the chain link style load cell is not practical for two reasons. First, the number of parts of line which are reeved through the hook block are constantly being changed by the operator in the field to correspond to the load being lifted. Accordingly, if the number of parts of line are not an even number, the load bearing cable will not have a termination point, and the chain link style load cell cannot be used. Secondly, even if the chain link style load cell were used, it would be prohibitively expensive to get the signal from the load cell from the end of the telescoping boom to the operator.
Load pins are transducers which are designed to measure the forces being transferred through the pin. They are most effective when used with cable sheaves. The sheaves tend to equalize the torsional forces that would otherwise cause a large hysteresis if, for example, the load pin was used as one of the load bearing cylinder pins. Load pins face the same problem as the load shackle and chain link style load cells in that it is prohibitively expensive to transmit the signal from the load pin down a telescoping boom to the operator.
Microcell sensors are also available for measuring the load. However, these microcells are designed to be applied to the outside of a structure and exposure to the environment is unavoidable. In particular, these microcells are very sensitive to changes in temperature, especially changes which are caused by exposure to direct sunlight. Accordingly, in the crane environment, the use of microcells can produce unreliable weight indications.
In U.S. Pat. No. 4,039,084, the stress in a crane lifting hydraulic cylinder is determined by four strain sensors which are mounted on the exterior of the hydraulic cylinder piston rod or on a supporting means attached to the end of the piston rod. The problems with this device are that a plurality of sensors are required and each of the sensors is mounted such that they are exposed to the environment. Thus, continued exposure to rain, snow and sunlight can deteriorate the its sensing capabilities. In addition, when the sensors are exposed to direct sunlight, the temperature difference between the sensor and its surrounding environment can also result in erroneous sensor indications.